Chemistry Reference
In-Depth Information
Our results on a bilayer showed that the standard coarse grained model can lead
to a much more diverse behavior than previously reported [
189
]. Figure
15
shows
the temporal evolution of a typical PE monolayer when the system is brought into
contact with a solution of PE of opposite sign via successive dipping-rinsing sub-
cycles with the same type of PE. Values for the main system parameters are
roughly similar to previous works of Patel et al. [
188
,
189
]: the fraction degree of
charge is
f
1/3 (i.e., one in each three monomers is charged), the density
surface charge is set to
¼
S ¼
Q
tot
/A
¼
0.5625, and both attractive short-range
interactions (monomer-monomer
e
m
s
)areof
the order of
k
B
T
. Our simulations revealed that the polyanions and the polycations
can complex with each other, thus destroying the bilayer and the build-up of a
PEM. We observed two different modes of complexation in our model: at low
values of the monomer-monomer interaction
e
m
m
~ 0.2, the bilayer expels just
small complexes consisting of only a few pairs of chains. However, when the
monomer-monomer attraction is of the order of the thermal energy,
e
m
m
and monomer-substrate
e
m
m
~1,the
oppositely charged PEs do form a complex followed by a dewetting of
the surface. Patel et al. [
189
] had already reported that for some longer runs the
PEMs surface coverage also gradually decreased. They unfortunately only pre-
sented results for PEMs with six and eight layers, but not for bilayers. Thus it is
plausible that the PEMs in the study of Patel et al. [
189
] corresponded to kineti-
cally trapped states, which given enough time and a bulk reservoir large enough to
accommodate the adsorbed matter might also redissolve over the course of time.
However, we also found that if to the (unstable) bilayer we added sufficiently fast
a third PE layer, then the trilayer seemed to be stable in the course of a very long
MD run. Also the subsequent addition of a fourth layer did not change the
apparent stability, at least within the long run time up to the order of tens of
millions of MD steps we could afford. This dewetting process has indeed been
observed in some types of PEMs, although it is poorly reported in the experimen-
tal literature because they focus mainly on reporting systems and pathways that
are able to form PEMs. In principle our findings could also be related to the
experimental fact that odd numbers of layers are more stable than even numbers
of layers.
To investigate further the critical role played by dispersion forces for the PE
adsorption process and for PEMs stability, we performed both CG and atomistic
simulations. To investigate the role of dispersion forces in the CGmodels we took
already formed PEMs and progressively reduced the value of the monomer
charges
q
from 1 to 0. We observed that the PEM is able to sustain its inner
structured layering almost without modifications even when electrostatic inter-
actions are quite drastically lowered. Only for
q
0.2 could noticeable changes
in the inner PEM structure be observed. Therefore, it seems that there is a
minimum level of electrostatic interaction needed to sustain a PEM structure,
but once such a level is attained, the inner structure is almost no further modified
by increasing the strength of electrostatic interaction among PEs. This was
verified by increasing the charge values up to three times the usual unit charge
of monomers.
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